Gauges for thickness measurement purposes are commonly employed tools in many industries. Typically, such gauges include a pair of members which contact opposed sides of the workpiece being measured and a measurement readout device which provides a reading of the distance between the two elements, and thus a reading of the workpiece thickness.
It is often desired to determine "residual thickness". This term as used herein is meant to be the thickness of a portion of a workpiece which has had a groove or other similar indentation placed therein. For example, in forming easy-open metallic container ends, a scoreline is formed in a portion of the end and the residual score thickness is defined as the thickness of metal remaining within the scoreline. Accurate control of such thickness is necessary, since, if the residual score thickness is too great, there will be difficulty in opening the container. Conversely, if the residual score thickness is too small, premature failure of the score may occur, resulting in leakage.
When measuring residual thickness in grooves, a constraint often results from the narrow width of such grooves. For example, scorelines in container end walls often have widths in the order of 0.0015 inches (0.0038 centimeters). Additionally, such measurements must often be taken at positions of curvature, further limiting the ease of entering the groove. Thus, it has been common in the past to employ thickness gauges wherein the measurement element entering the groove comprised a thin, pointed element.
Accuracy problems arise when employing such a pointed measuring element. When measuring the thickness of workpieces formed from such materials as plastics, aluminum alloys and the like, which materials are fairly soft, penetration by the pointed measurement element of the workpiece could occur. Such penetration produces an inaccurately thin reading. For example, aluminuous metal container ends have score residual thickness in the order of 0.0045 inches (0.0114 centimeters). A 0.0002 inch (0.00051 centimeter) penetration of the workpiece in such a case results in approximately a 4.5% error.
It is thus the primary object of the present invention to provide a thickness gauge, especially designed for measuring residual thickness in grooves, in which the element entering the groove is designed to permit ease of entry at curved positions along the groove, and in which the grove entering element substantially reduces the chances for penetration of the workpiece, thereby increasing accuracy of the reading.